1. Field of the Invention
This invention relates to a radiant brazing apparatus and process wherein the heater assembly includes a sensor for rapidly measuring the temperature of the section of the conduit to which heat was applied.
2. Description of the Prior Art
Devices and processes for applying a brazing heat to the inside surfaces of conduits are known in the prior art. Such processes are used to braze reinforcing sleeves within the heat exchange tubes of nuclear steam generators. In such prior art processes, a tubular sleeve which is circumscribed at each end by a ring of brazing alloy is inserted into a tube in need of repair and slidably positioned across the section of the tube where the walls are in a damaged condition due to corrosion or denting. The ends of the tube surrounded by the rings of brazing alloy are then usually hydraulically expanded so that they snugly engage the inner walls of the tube. To complete the joint, a heater assembly formed from an electrical resistance wire coiled around a mandrel is slid up the sleeve and actuated so that the heat generated by the wire melts the rings of brazing alloy disposed between the outer wall of the sleeve and the inner wall of the tube, thereby forming a pair of watertight braze joints between the ends of the sleeve and the tube. Such prior art sleeving operations are frequently performed in the longitudinal sections of the heat exchange tubes which extend through the tubesheet of the nuclear steam generator due to the tendency of the walls of these tubes to corrode and dent in this region.
While the aforementioned one-step brazing process frequently resulted in watertight braze joints, problems sometimes arose when sludge deposits became tightly wedged in the annular space between the walls of these tubes and the bores or other openings in the tubesheets and support plates through which these tubes extended. Under such circumstances, the sludge deposits sometimes bound the outer walls of the tube so tightly against the walls of the bores and the tubesheets and support plates that the tubes were prevented from thermally expanding along their longitudinal axis when the electrical resistance coil of the heater assembly applied a brazing heat to the tube and the sleeve. Consequently, some of these tubes expanded radially rather than longitudinally in the area where brazing heat was applied. Such a radial expansion in the area of the braze joint sometimes created undesirable gaps in the joint which jeopardized the integrity of the watertight seal the braze joint was intended to produce. The extent to which such undesirable gapping occurred would, of course, vary widely between any group of sleeve tubes due to the varying amount of binding forces the sludge deposits applied to these tubes when a brazing heat caused them to thermally expand. However, in those tubes where sludge deposits applied a substantial amount of resistance to the longitudinal tube expansions, the amount of gapping which occurred significantly jeopardized the ability of the braze joint to create a watertight seal.
In order to overcome the shortcomings of such one-step brazing processes in consistently creating watertight braze joints, a two-step process was developed which is specifically described and claimed in U.S. patent application Ser. No. 634,336, filed July 13, 1984, invented by John M. Driggers and assigned to Westinghouse Electric Corporation. In the first step of this process, the heater assembly is slid along the longitudinal axis of the tube being repaired to a region which is close to, but which does not include, the region of the sleeve and tube where the ring of brazing alloy is situated. The electrical resistance coil of the heater assembly is then actuated. In tubes where the binding forces will not allow the tube to longitudinally expand in response to heat, the heater assembly will thermally create a radial expansion in the region of the tube adjacent the electrical resistance coil. In the second step of the process, the heater assembly is slid up the sleeve to a position where the electrical resistance coil of the heater assembly is adjacent the ring of brazing material which circumscribes the reinforcing sleeve, while electrical power continues to flow through the coil. The residual tensile stress created by the first, thermally induced radial expansion in the tube avoids the creation of gaps in the braze joint between the sleeve and inner walls of the tube. This process has been particularly effective in creating high-quality braze joints and sleeving operations in nuclear steam generators.
Unfortunately, while this two-step brazing process constitutes a substantial advance in the art over the one-step process in creating high quality gapless braze joints, proper execution of this process is sometimes difficult due to the wide variation in the heat-sink properties of the particular sleeve/tube combination being brazed. A principal cause of these heat-sink variations is the amount of dark colored oxides on the outside walls of the tube being brazed; the presence of a layer of such dark oxides over the outer walls of the tube can greatly increase the radiant heat losses out of the sleeve/tube combination due to the phenomenon of black-body radiation. Other causes of heat-sink variations include the amount and type of corrosion between the tube and the tubesheet (some oxides are fairly good heat conductors), as well as the amount of water surrounding the tube being brazed. These variations make it difficult to consistently apply the proper amount of heat to the tube so that the heater assembly induces the required amount of thermal radial expansion in the first step of the process, and then proceeds to properly fuse the brazing alloy in the second step so that the desired high-quality joint is created. While both the one-step and two-step processes can, for the most part, be effectively executed by actuating the resistance coil of the heater assembly at a power level and for an amount of time selected on the basis of the "average" heat-sink properties of such sleeve/tube combinations, variations in these properties can occasionally result in the application of an insufficient or excessive amount of heat to the joint. Either extreme can, of course, jeopardize the integrity of the resulting joint.
Clearly, there is a need for some means for determing whether or not the proper amount of heat has been applied to a particular sleeve/tube combination in order to create a high-quality braze joint. Ideally, such a sensor should be mounted on or near the heater mandrel, and capable of quickly and accurately determining the temperature of the section of the sleeve heated.